1. Field of the Invention
This invention relates to a recording apparatus, and more particularly, to control of recording current when recording a digital signal on a magnetic tape using a magnetic head.
2. Description of the Related Art
A digital VCR (video cassette recorder) for recording a digital signal on a magnetic tape using a magnetic head is known as an apparatus for recording a signal on a recording medium.
FIG. 1 is a schematic block diagram of circuitry described in "LSI/IC for DCC", National Technical Report, Vol. 39, No. 6, December 1993, as an example of a magnetic-head driving circuit for controlling recording current passing through a head in such a digital VCR. In FIG. 1, a series circuit comprising a switch 212, controlled by an output from an inverting circuit 225, and a first constant-current source 214, which can be turned on and off, and a series circuit comprising a switch 216, controlled in accordance with recording data, and a second constant-current source 218, which can be turned on and off, are connected in parallel to a power supply 210 having an output voltage V.sub.cc, and a magnetic head 224 is connected between a node 220, where the switch 212 and the constant-current source 214 are connected, and a node 222, where the switch 216 and the constant-current source 218 are connected.
FIG. 2 illustrates the relationship between the values of digital data to be recorded, and the operations of the switches 212 and 216 and the constant-current sources 214 and 218. That is, in the circuitry shown in FIG. 1, when the value of data to be recorded is "1", the switch 212 is switched on, the switch 216 is switched off, the constant-current source 214 is turned off, and the constant-current source 218 is turned on, so that a constant current I.sub.c determined by the constant-current source 218 flows from the plus terminal to the minus terminal of the power supply 210 via the switch 212, the magnetic head 224 and the constant-current source 218. Accordingly, the current I.sub.c passes through the magnetic head 224 from the node 220 toward the node 222. On the other hand, when the value of data to be recorded is "0", the switch 212 is switched off, the switch 216 is switched on, the constant-current source 214 is turned on, and the constant-current source 218 is turned off, so that the constant current I.sub.c determined by the constant-current source 214 flows from the plus terminal to the minus terminal of the power supply 210 via the switch 216, the magnetic head 224 and the constant-current source 214. Accordingly, the current I.sub.c passes through the magnetic head 224 from the node 222 toward the node 220.
If the current passing through the magnetic head 224 from the node 220 toward the node 222 is assumed to be positive, statically, a recording current I.sub.rec passing through the magnetic head 224 equals +I.sub.c and -I.sub.c when data to be recorded is "1" and "0", respectively.
As is well known, when a constant current passes through an inductive load, such as the magnetic head 224 or the like, the following voltage e appears between terminals of the load for a change in the passing current i: EQU e=-L(di/dt)(V),
where L is the inductance of the load.
In the circuitry shown in FIG. 1, if it is assumed that the switches 212 and 216 are ideal and their on-resistance equals zero, negative transient pulse voltages as shown in FIGS. 3(b) and 3(c) are generated. FIG. 3(a) illustrates the waveform of the recording current passing through the magnetic head 224. The output voltage V.sub.cc of the power supply 210 must be selected so as to provide pulse voltages corresponding to the inductance L of the magnetic head 224, the transfer rate of digital data to be recorded, and the value of the recording current. For example, if the output voltage V.sub.cc of the current supply 210 is too low, pulse voltages at the nodes 220 and 222 are too low, as shown in FIGS. 4(b) and 4(c), respectively, so that rise and fall of the recording current are not sharp, as shown in FIG. 4(a). Normal recording cannot be performed in this state.
Recently, camcorders, in each of which a VCR and a video camera are integrated, have been rapidly popularized, so that video recording is ordinarily and easily performed outdoors using a battery as a power supply. When providing a digital VCR unit, it is desired to reduce power consumption in order to increase the recordable time period when the battery of the VCR unit is in a fully charged state.
However, the above-described conventional magnetic-head driving circuit has the following problems. That is, a high voltage for providing pulse voltages required at the nodes 220 and 222 is always applied to one of the constant-current sources 214 and 218, thereby increasing power consumption in the constant-current sources 214 and 218, and resulting in a short recording time period in the case of battery drive.